Device and method for blood treatment with single needle operation

ABSTRACT

The present invention relates to a device and a method for blood treatment in a single-needle operating mode. The blood treatment device has an extracorporeal blood circuit comprising a blood supply line leading to the inlet of a blood treatment unit and a blood return line leading away from the outlet of the blood treatment unit. Blood collecting means, for example a blood collecting container, is provided in the blood return line, said means being connected to means for storing gas, for example an air reservoir, by a flow line. The flow connection comprises two line branches, one branch being used to transfer gas from the air reservoir into the blood collecting container and the other branch being used to transfer gas from the blood collecting container into the air reservoir. The connection branch for transferring gas from the air store into the blood collecting container contains means for compressing air, for example a compressor. The device according to the present invention may be operated in such a way that the filling level of the means for collecting blood can be determined as precisely as possible.

FIELD OF THE INVENTION

The present invention relates to a device for blood treatment forsingle-needle operation, which comprises an extracorporeal blood circuitwhich has a blood supply line leading to the inlet of a blood treatmentunit and a blood return line departing from the outlet of the bloodtreatment unit, whereby the blood supply line and blood return line leadto a common needle (cannula) and, respectively, lead away from a commonneedle. Furthermore, the present invention relates to a method for bloodtreatment in the single-needle operation.

BACKGROUND

Blood treatment apparatuses with a blood treatment unit through which apatient's blood flows are generally known. These include, for example,the known hemodialysis, hemofiltration or hemodiafiltration apparatuses.The known blood treatment apparatuses can be operated in thesingle-needle or double-needle operation.

With the double-needle technique, the blood is drawn off from thepatient's blood vessel via a first needle, conveyed into the bloodtreatment unit of the blood treatment apparatus and returned into thepatient's blood vessel via a second needle. For the removal and returnof the blood, use is made of exchangeable hose systems with a bloodsupply line and blood return line, to which the two needles areconnected. The hose systems, which are to be thrown away, are alsoreferred to as disposables.

With the single-needle technique, the removal and return of the bloodtakes place via a single needle. The blood removed from the patient isstored in a reservoir during an arterial phase, in order then to bereturned in a venous phase from the storage unit into the patient'sblood system through the same needle.

A blood treatment apparatus for the single-needle operation is knownfrom EP-A-0 472 480 B1. In an embodiment of the known blood treatmentapparatus, two blood expansion chambers are provided for temporarystorage of blood, which are disposed upstream and downstream of theblood treatment unit. The blood treatment apparatus has a controldevice, which keeps the pressure in the expansion chambers essentiallyconstant. Level sensors are provided in order to detect the level of theliquid in the expansion chambers.

DE 10 2005 001 779 A1 describes a set for a disposable for the operationof a blood treatment apparatus in the single-needle or double-needleoperation. Apart from the blood supply line and blood return line forthe connection to the blood treatment unit, the disposable comprises anexpansion unit which can be coupled to the air separation unit in orderto increase the volume for the single-needle operation. During thesingle-needle operation, blood is conveyed through the blood supply lineinto the blood treatment unit and out of the blood treatment unit intothe air separation and expansion unit in the arterial phase, the bloodsupply to the patient being interrupted. A predetermined pressure isbuilt up in the air separation and expansion unit, which is monitored bymeans of a pressure measuring unit. By means of a compressed air unit, apredetermined pressure can be established in the expansion and airseparation unit by actuating an air pump which is incorporated between atank and the expansion unit. Moreover, it is proposed to calculate theblood volume in the expansion and air separation unit with the aid ofthe measured values from three pressure sensors and the known systemvolumes. Furthermore, it is proposed to use the air pump to regulate thepressure in the venous phase, so that the delivery rate of the blood canbe adapted in the optimum fashion.

An object of the present invention is to provide a device for bloodtreatment for the single-needle operation, wherein the blood volume inthe blood reservoir can be ascertained without level sensors and thebehavior in the case of malfunction can be improved. Moreover, it is anobject of the present invention to provide a blood treatment method inthe single-needle operation, wherein the blood volume in the bloodreservoir can be ascertained without level sensors and the behavior inthe case of malfunction can be improved.

SUMMARY

The device for blood treatment according to an example embodiment of thepresent invention comprises an extracorporeal blood circuit, which has ablood supply line leading to the inlet of a blood treatment unit and ablood return line departing from the outlet of the blood treatment unit.This extracorporeal blood circuit with the blood treatment unit does nothave to be part of the blood treatment device, but may be a disposableintended for single use, which is placed into the treatment unit for theblood treatment.

Moreover, the blood treatment device comprises means for collectingblood and means for storing gas, in particular air. The means forcollecting blood and for storing gas are in each case a reservoir whichforms a closed-off volume, for example a blood and air chamber with apredetermined volume.

The means for collecting blood is connected to the means for storing gasso that, in the arterial phase during the filling of the means forcollecting blood, air expelled from the means for collecting blood canpass into the means for storing gas and, in the venous phase, air fromthe means for storing gas can pass into the means for collecting blood,as a result of which the blood previously collected in the arterialphase is expelled from the means for collecting blood. The effect ofthis is that blood is removed from the patient in the arterial phase andthe patient's blood treated in the blood treatment unit is returned backto the patient in the venous phase.

The connection between the means for collecting blood and the means forstoring gas comprises a first and second connection path. The firstconnection path connects the means for collecting blood to the means forstoring gas in such a way that, in the arterial phase, air expelled fromthe means for collecting blood is transferred to the means for storinggas, means for interrupting the connection being provided in the firstconnection path. The second connection path connects the means forcollecting blood and storing gas in such a way that, in the venousphase, air stored in the means for storing gas is transferred to themeans for collecting blood. The second connection path contains meansfor compressing gas, so that the air stored in the means for storing gascan be transferred into the means for collecting blood. With the meansfor compressing the air, a predetermined pressure can be built up in themeans for collecting blood both during the arterial and the venousphase. The means for compressing gas can be designed for example as aconventional compressor.

The means for collecting blood and storing gas form together with thefirst and second connection path a closed-off volume, into which no gascan enter and from which no gas can exit. The closed-off volume isventilated/deaerated solely for the purpose of initializing the system.The first connection path and the second connection path can be formedby separate lines. The first and second connection paths can howeveralso comprise a common line segment.

An air infusion into the patient can be eliminated with greaterreliability by the fact that there is established in the means forstoring gas a pressure which in no case of operation is greater than thepressure that is established in the means for collecting blood. Theeffect of this is that air can pass from the means for storing gas intothe means for collecting blood solely with the means for compressinggas. In the event of failure of the means for compressing gas, forexample a leakage of the compressor, no air can on the other hand passinto the means for collecting blood. The situation is thus eliminatedwhere the means for collecting blood run empty unintentionally, as aresult of which air could pass into the patient.

In practice, an attempt is made to ensure that the return pressure withwhich blood is expelled from the means for collecting blood can beadjusted as low as possible, whereas the stroke volume is as large aspossible. In order to be able to satisfy the condition that, in theevent of failure of the compressor, air cannot pass from the means forstoring gas into the means for collecting blood with a low returnpressure and a large stroke volume, the system for initializing thesuccessive arterial and venous phases is relieved to a predeterminedpressure, preferably the ambient pressure, at the time of thechange-over from the arterial to the venous phase, i.e., at the time ofthe upper change-over point at which the means for collecting blood arefilled. In the successive arterial and venous phases, therefore, thepressure in the means for storing gas always lies below thepredetermined pressure, in particular the ambient pressure, to which theclosed off-system has been relieved.

During the arterial phase, the means for storing gas are not in directcontact with the patient, since the extracorporeal blood circuit isseparated on the venous side, for example with a hose clamp. The courseof the pressure in the means for storing gas thus plays a rather lesserrole for patient safety during the arterial phase. It is decisive,however, that at the end of the arterial phase, i.e., at the start ofthe venous phase, the desired return pressure is present in the meansfor collecting blood. This is preferably achieved by the fact that thearterial phase is divided into two time intervals. In a preset firsttime interval, a connection is made between the means for collectingblood and the means for storing gas, so that the air expelled during thefilling of the means for collecting blood passes into the means forstoring gas, as a result of which a specific pressure is built up there.In a preset second interval, however, the connection between the meansfor collecting blood and storing gas is interrupted. The pressure in themeans for storing gas thus remain constant, whereas the pressure in themeans for collecting blood rises further, since blood is continuouslyconveyed into the means for collecting blood in the arterial phase. Twoseparate air volumes thus result.

In a preferred embodiment of the blood treatment device according to thepresent invention, the second connection path between the means forstoring gas and collecting blood comprises a line for conveying gas,which connects the means for storing gas to the means for collectingblood. The means for compressing gas, for example a compressor, aredisposed in this line. The first connection path comprises a bypass linebypassing the means for compressing gas, in which bypass line a bypassvalve is disposed. When the means for compressing gas are not operated,the closed-off volume of the means for collecting blood are separatedfrom the closed-off volume of the means for storing gas when the bypassvalve is closed. When the bypass valve is opened, on the other hand, thetwo volumes form a common closed-off volume.

In a further preferred embodiment, the line for conveying gas comprisesa first line segment, which connects the means for collecting blood tothe pressure side of the means for compressing gas, and a second linesegment, which connects the suction side of the means for compressinggas to the means for storing gas.

For the relief of the closed-off volume to the ambient pressure duringthe initialisation of the system, means for ventilation or deaerationare preferably provided in the first line segment.

The means for collecting blood are preferably designed as a containerwith a predetermined volume, which has an inlet and an outlet and isdisposed in the blood return line of the extracorporeal circuit. Themeans for storing gas are preferably designed as a container with apredetermined volume.

The blood in the extracorporeal circuit is preferably conveyed by meansof a blood pump which is disposed in the blood supply line.

A particular advantage of the blood treatment apparatus according to thepresent invention lies in the fact that the level of the means forcollecting blood can be ascertained as accurately as possible withoutlevel sensors solely on the basis of pressure measurements. A preferredembodiment of the blood treatment device according to the presentinvention comprises means for measuring the pressure in the closed-offvolume of the means for collecting blood, the closed-off volume in theconnection paths between the means for collecting blood on the one handand the means for compressing gas on the other hand and the closed-offvolume of the means for storing gas, the blood volume being calculatedfrom the measured pressure values.

In order to determine the pressure in the connection paths between themeans for collecting blood on the one hand and the means for compressinggas on the other hand, the pressure can be measured either in a linesegment of the first connection path and/or in a line segment of thesecond connection path, wherein the line segments extend between themeans for collecting blood on the one hand and the means for compressinggas or the means for interrupting the connection on the other hand. Ifthe first and second connection paths have a common line segment, it isalso possible to measure the pressure in the common line segment of thetwo connection paths with a pressure sensor.

An example of embodiment of the present invention is explained ingreater detail below by reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of embodiment of the blood treatment deviceaccording to an example embodiment of the present invention for thesingle-needle operation in a very simplified schematic representation,

FIG. 2 shows the course of the level and the pressure during theinitialization and the operation of the blood treatment device accordingto the example embodiment of the present invention, and

FIG. 3 shows the course of the pressure during the successive arterialand venous phases after the initialization.

DETAILED DESCRIPTION

FIG. 1 shows the main components of a blood treatment device, inparticular a dialysis apparatus, for the single-needle operation in aschematic representation.

During the blood treatment, the dialysis apparatus comprises anextracorporeal blood circuit 1, which includes a blood treatment unit 2,for example a dialyser, designed as a disposable. Dialyser 2 is dividedby a semipermeable membrane 3 into a blood chamber 4 and a dialysingfluid chamber 5.

In the extracorporeal blood circuit, the blood is conveyed by means of ablood pump 6, which is part of the dialysis apparatus. The dialysingfluid circuit is not represented in FIG. 1.

Inserted into the dialysis apparatus is a hose set 7, which is disposedof after the treatment. Disposable 7 has a blood supply line 8 leadingto inlet 4A of blood chamber 4 of dialyser 2, said blood supply linebeing inserted into roller pump 6 of the dialysis apparatus, and a bloodreturn line 9 departing from outlet 4B of the blood chamber. Bloodsupply line and blood return line 8, 9 are connected to a common cannula10 (needle).

Disposed in blood return line 9 of the disposable are means 11 forcollecting blood, which are designed as a container with a predeterminedvolume.

The means for collecting blood are referred to in the following as ablood collecting container or blood storage unit.

Means 12 for interrupting the blood return line, for example a venoushose clamp, are disposed downstream of blood collecting container 11 onblood return line 9.

Blood collecting container 11 has an inlet 13, to which a first segment9A of blood return line 9 leads, and has an outlet 14, from which asecond segment 9B of blood return line 9 departs. For the detection of aspecific level in blood collecting container 11, the dialysis apparatushas a level indicator 15, which detects when the level in the containerreaches a preset value. A pressure transmitter 16 is also provided,which measures the pressure in blood collecting container 11.

When blood collecting container 11 is filled with blood, a specific airvolume remains above liquid level 17 in the blood storage unit. Theblood storage unit is in a flow connection with means 18 for storinggas, in particular air, which are designed as a container with aclosed-off volume. Means 18 for storing gas are referred to in thefollowing as an air storage container or air storage unit.

In order that the blood storage unit and air storage unit cancommunicate with one another, a line 19 which leads to air storage unit18 departs from the upper side of blood storage unit 11. Disposed inline 19 is means 20 for compressing gas, which can be designed forexample as a conventional compressor. As long as the compressor is notbeing operated, the compressor interrupts the flow connection betweenthe blood storage unit and the air storage unit. When the compressor isoperating, on the other hand, air present in the air storage unit istransferred into the blood storage unit. Since the air is compressed, apredetermined pressure is built up in the blood storage unit.

Line 19 has two line segments 19A, 19B, whereof one line segment 19Aconnects blood storage unit 11 to pressure-side connection 20A ofcompressor 20 and other line segment 19B connects suction-sideconnection 20B of compressor 20 to air storage unit 18. These linesegments 19A, 19B form a connection path for the transfer of gas out ofthe air storage unit into the blood storage unit.

In order to be able to transfer air from the blood storage unit into theair storage unit when the compressor is not in operation, a bypass line21 is provided which departs from first line segment 19A of line 19 andleads to second line segment 19B of line 19. A bypass valve 22 isincorporated into bypass line 21. Together with the corresponding linesegments of line 19, bypass line 21 forms a connection path forconveying gas out of the blood storage unit into the air storage unit.

In order to prevent liquid from passing from the blood storage unit intothe air storage unit, there is disposed in first line segment 19A ofline 19 a filter 23, which contains a hydrophobic membrane, i.e.permeable to air, but impermeable to liquid. Since the blood storageunit is filled only up to a maximum level, however, liquid can in anycase only pass into line 19 in the event of a malfunction.

For the ventilation/deaeration of the closed-off volume, which includesthe blood storage unit and the air storage unit as well as line 19,means 24 is provided for ventilation/deaeration, which comprise aventilation/deaeration line 24 with a ventilation/deaeration valve 24B,said ventilation/deaeration line being connected for example to firstline segment 19A of line 19. Ventilation/deaeration line 24A can inprinciple depart from any point of the volume to be ventilated ordeaerated. The ventilation/deaeration should in particular take place inthe part on the machine side.

Apart from pressure transmitter 16 for measuring the pressure in theblood storage unit, a pressure transmitter 25 is provided for measuringthe pressure in first line segment 19A of line 19 between filter 23 andcompressor 20 and a further pressure transmitter 26 is provided formeasuring the pressure in air storage unit 18. A temperature sensor Tfor measuring the temperature of the air in the air storage unit isprovided in the air storage unit.

The dialysis apparatus has a central control and computing unit 27,which is connected via electrical lines (not shown) to blood pump 6,venous hose clamp 12, bypass valve 22, ventilation/deaeration valve 24B,level indicator 15, compressor 20 and pressure transmitters 16, and 26.The central control and computing unit is in turn connected to an inputunit (not shown), which creates an interface between the user and themachine.

The operation of the dialysis apparatus is described below in detail byreference to FIGS. 2 and 3. Central control and computing unit 27controls the dialysis machine as follows.

At the start of the actual dialysis treatment, an initialization of thesystem takes place with the following process steps.

FIG. 2 shows the level in the blood storage unit as a function of timeduring the individual phases of the initialization. Moreover, FIG. 2shows the course of the pressure in the blood storage unit, which isreferred to as the chamber pressure, the pressure in the air storageunit, which is referred to as the storage-unit pressure, and thepressure in first line segment 19A of line 19, which is referred to asthe line pressure. Furthermore, FIG. 2 shows the time-related course ofthe total air mass enclosed in the blood storage unit and in the airstorage unit as well as the corresponding segments of lines 19, 22.

In the first initialization step, the blood level in the blood storageunit is lowered below a specific level, which lies between thechange-over point, at which the change-over is to take place from thearterial phase to the venous phase during the operation of the dialysisapparatus, and the lower change-over point, at which the change-overfrom the venous to the arterial phase is to take place. For thispurpose, control and computing unit 27 opens venous hose clamp 12 withblood pump 6 standing still and puts compressor 20 into operation untilthe desired liquid level is reached, which is detected by the levelindicator. FIG. 2 shows that the level falls, whilst the total air massin the system remains constant. This step can be skipped if the bloodlevel is already located below the desired level.

In the second step, the blood level is then adjusted to the desiredlevel, which is detected by the level indicator. For this purpose,bypass valve 22 and ventilation/deaeration valve 24B is opened and bloodpump 6 is operated with venous hose clamp closed until the desired levelis reached. FIG. 2 shows that the level rises to the desired level,whilst the air mass in the system diminishes.

When the desired level is reached, there is a waiting time until thechamber pressure and storage-unit pressure, as well as the linepressure, have become adjusted to ambient pressure. The level therebyremains constant, whilst the air mass continues to diminish slightly(step 3). Only then is ventilation/deaeration valve 24B closed again(step 4).

The blood storage unit then continues to be filled with blood. With thebypass valve opened, blood pump 6 is operated until such time as thelevel in the blood storage unit has reached the level of the upperchange-over point (step 5). The air mass in the system thereby remainsconstant. Since the chamber and storage-unit pressure and the linepressure are measured by means of the pressure transmitters, the levelin the blood storage unit may be continuously calculated. The controland computing unit calculates the level in the blood storage unit andstops the blood pump when the level has reached the level of the upperchange-over point. This will be described in detail below.

After the control and computing unit has stopped blood pump 6,ventilation/deaeration valve 24B is opened again, so that the pressurebuilt up in the system is relieved to the ambient pressure (step 6).FIG. 2 shows that the level remains constant, whilst the chamber andstorage-unit pressure as well as the line pressure fall to ambientpressure. Since the volume that is enclosed by the system, i.e. both thevolume of the blood storage unit and the air storage unit as well as thelines, and also the pressures in the system are known, the air masscontained in the system can be calculated. This will be described indetail below.

Ventilation/deaeration valve 24B is closed as the last step of theinitialisation, whereby neither the level nor the pressures and the airmass in the system change (step 7). The ventilation/deaeration valveremains closed throughout the whole blood treatment unless a renewedinitialization is required, for example after the detection of an airleak. The initialization is thus completed, and the blood treatmentstarts with the first venous phase.

In the first venous phase, compressor 20 is operated with bypass valve22 closed, venous hose clamp 12 being opened and blood pump 6 standingstill. During the operation of the compressor, air from storage unit 18is compressed and fed to blood storage unit 11. The chamber pressure andline pressure thus increase, whereas the storage-unit pressurediminishes. At the same time, the level in the blood storage unitdiminishes continuously, until the level of the lower change-over pointis reached. It is decisive here that the storage-unit pressure liesbelow the chamber pressure and therefore also below the line pressure,in order to prevent an entry of gas from the air storage unit into theblood storage unit in the event of a malfunction. In addition, anattempt is made to ensure that the storage-unit pressure lies even belowthe ambient pressure.

The arterial phase then begins, in which the blood storage unit is againfilled with blood taken from the patient, this again being followed bythe venous phase in which the blood is fed from the blood storage unitback to the patient.

The air storage unit is dimensioned with a size such that sufficient airis also present in the system at the end of the venous phase, in orderto be able to maintain the desired return pressure in the blood storageunit. In order to be able, with the same initialisation, to adjust allthe operating points with a return pressure of 0 to 500 mmHg relativewith a stroke volume up to 60 ml, an air storage unit with a storagevolume of approx. 300 ml is required in practice.

The time-related course of the chamber pressure and storage-unitpressure as well as the line pressure during the actual arterial andvenous phases after the initialisation of the system is represented inFIG. 3, which shows a detail from FIG. 2.

Blood pump 6 is operated throughout the whole arterial phase, compressor20 standing still. Venous hose clamp 12 remains closed throughout thewhole arterial phase.

At the start of the arterial phase, control and computing unit 27 opensbypass valve 22, so that the air expelled out of blood storage unit 11passes via bypass line 21 into air storage unit 18. The storage-unitpressure therefore increases, whilst the chamber and line pressure atfirst falls, after which it also then rises, as it were, with thestorage-unit pressure. The air mass contained in the blood storage unitand in the respective line volume therefore constantly diminishes.

As soon as the air mass contained in the blood storage unit and the linevolume has reached a predetermined amount, which results from thedesired stroke volume and the desired return pressure, the control andcomputing unit closes the bypass valve. Two separate air volumes thusarise, i.e. the air volume of the blood storage unit with the respectiveline segments and the volume of the air storage unit with respectiveline segments. The blood pump is then operated with the bypass valveclosed, so that the storage-unit pressure remains constant, whilst theair in the blood storage unit and the respective line volume iscompressed until, upon reaching the desired stroke volume, the desiredreturn pressure is also reached. FIG. 3 shows that the chamber and linepressure have risen to the desired return pressure at the end of thearterial phase, whereby the storage-unit pressure throughout the wholearterial phase always lies below the chamber and line pressure, inparticular below the ambient pressure. This thus rules out air passingfrom the air storage unit into the blood storage unit even in the eventof a malfunction of the system, for example with a leakage of thecompressor.

Instead of the division of the arterial phase into a first and secondtime interval, other embodiments are also possible with which theadvantages according to the present invention can also be achieved. Analternative embodiment makes provision to use, instead of bypass valve22, a pressure-controlled valve which opens when a threshold pressurecorresponding to the return pressure is reached, so that a constantpressure prevails in the blood storage unit in a certain time period.

The control and computing unit then changes over to the venous phase,whereby the bypass valve remains closed, the blood pump is stopped andthe compressor put into operation and the venous hose clamp is opened.The compressor is operated throughout the whole venous phase, the bloodpump standing still. The venous hose clamp remains open and the bypassvalve closed during the venous phase.

The compressor conveys air out of the air storage unit into the bloodstorage unit in order to build up an excess pressure, so that blood isconveyed from the blood storage unit. The compressor is operated in sucha way that the desired return pressure is established in the bloodstorage unit. Since air from the air storage unit is continuously fed tothe blood storage unit, the storage-unit pressure continuouslydiminishes. The decisive factor again is that the storage-unit pressurealways lies below the chamber and line pressure, preferably also belowthe ambient pressure, so that an air infusion out of the air storageunit into the patient is ruled out in the event of a malfunction. Thevenous phase is ended when the blood level in the blood storage unit hasagain fallen to the level of the lower change-over point. The nextarterial phase then follows.

The calculation of blood volume V_(blood) present in the blood storageunit is described below, said calculation being carried out continuouslyor in preset intervals by the control and computing unit during theoperation of the dialysis apparatus. If the blood volume is known, thiscan be used for comparison with the change-over points from the arterialto the venous phase or vice versa.

In order to calculate blood volume V_(blood) present in the bloodstorage unit, it is first necessary to determine the air mass in theblood storage unit. After pressure equalisation has taken place, thefollowing holds:

${pV} = {\frac{m}{M_{n}}{RT}}$with:p=pressure (absolute), V=volume, m=mass, M_(m)=molar mass,R=general gas constant and T=temperature

Since M_(m) and R are constant and no absolute values are required forthe air mass, the latter do not need to be taken into accountexplicitly. Accordingly, only the following has to be determined:

$\frac{pV}{T} \equiv {m.}$

In order to determine the total air mass, the sum of all the partial airmasses, i.e. the air mass in the blood storage unit, the respectivelines and the air storage unit, must be calculated. For this purpose,all volumes filled with air must be multiplied by the given prevailingpressure and divided by the temperature.

$\left( \frac{pV}{T} \right)_{total} = {\frac{P_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}} \cdot V_{{blood}\mspace{20mu}{storage}\mspace{14mu}{{unit}/{air}}}}{T_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}} + \frac{P_{line} \cdot V_{line}}{T_{line}} + \frac{P_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}} \cdot V_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{T_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}}$  T_(blood  storage  unit) ≈ 273.15  K + 36  K$\mspace{20mu}{T_{line} \approx \frac{T_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}} + T_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{2}}$

The air volume of the blood storage unit V_(blood storage unit/air)changes during operation. If the blood level is at height H of the leveldetected by level indicator 15, the following holds:V _(blood storage unit/air) =V _(blood storage unit/air UT) −ΔV_(blood storage unit/UT-H).where V_(blood storage unit/air UT) is the air volume in the bloodstorage unit at lower change-over point UT andΔV_(blood storage unit/UT-H) is the volume difference between lowerchange-over point UT and the height of the level in the blood storageunit detected by level indicator 15.

At the end of the initialisation at upper change-over point OT, bloodwith the total stroke volume V_(stroke) is additionally present in theblood storage unit, so thatV _(blood storage unit/air) =V _(blood storage unit/air UT) −V_(stroke).

The remaining volumes remain constant. To a good approximation, thetemperatures can be put as constant if no temperature compensation takesplace. Preferably, however, a temperature sensor is provided for thetemperature measurement at least in the air storage unit, so that atemperature compensation can be carried out. Temperature sensors canhowever also be provided for other pressure values.

Delivered blood volume V_(blood) in the blood storage unit is calculatedwith each program loop during the whole cycle. Air volumeV_(blood storage unit/air) enclosed in the blood storage unit iscalculated with the aid of the measured pressures and the difference istaken between the air volume in the blood storage unit at lowerchange-over point UT and the air volume of the blood storage unit.

$V_{{blood}\mspace{14mu}{storage}\mspace{14mu}{{unit}/{air}}} = \frac{\left( \frac{pV}{T} \right)_{ges} - \frac{P_{line}V_{line}}{T_{line}} - \frac{P_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}V_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{T_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}}{\frac{P_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{T_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}}}$  V_(blood) = V_(blood  storage  unit/air  UT) − V_(blood  storage  unit/air).

The total air mass (pV/T)_(total) remains unchanged after theinitialization of the system, because the ventilation valve remainsclosed. Since the calculation of blood volume V_(blood) takes place withthe blood pump running or compressor running, a smoothing-out of thepressure signals and calculated blood volume V_(blood) is performed.

The change-over from the arterial to the venous phase (OT) takes placewhen the difference between calculated blood volume V_(blood) andadjusted stroke volume V_(stroke) is equal to zero, and the change-overfrom the venous to the arterial phase takes place when the calculatedblood volume is equal to zero.

It is described below how the control and computing unit calculates thetime at which, within the arterial phase, the change-over takes placebetween the first and second time interval of the arterial phase.

As already described above, the change-over point from the arterial tothe venous phase and back is possible by a simple comparison of theblood volume with the stroke volume, i.e. zero. The arterial phase isdivided, as already mentioned, into a first and a second time interval.In the first arterial phase, the blood pump conveys blood via thedialyser with the bypass valve open into the blood storage unit. Theunderpressure, which has previously built up in the air storage unit inthe venous phase, is used to assist the pump. In the second arterialphase, the air storage unit is decoupled from the remaining system byclosing the bypass valve, and the pressure in the blood storage unitincreases sharply due to the delivered blood volume. At the end of thesecond arterial phase, the desired setpoint pressure should be presentin the blood storage unit. The change-over point between the first andsecond arterial phase, therefore, must be selected in such a way thatthe still outstanding blood volume up to upper change-over point OTbuilds up the setpoint pressure in the blood storage unit and in theline.

Accordingly, it is necessary to calculate the air mass that builds upsetpoint pressure p_(setpoint) with compression on the air volumeavailable in the blood storage unit and the line at upper change-overpoint OT. The air mass in the blood storage unit and line is

${\frac{P_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}V_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{T_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}} + \frac{P_{line}V_{line}}{T_{line}}} = {\left( \frac{pV}{T} \right)_{total} - \frac{P_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}V_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{T_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}}$

At the change-over point from the first to the second time interval ofthe arterial phase, the air mass present in the blood storage unit andline must be equal to the air mass which is present in the blood storageunit and the line at upper change-over point OT.

$\left( \frac{pV}{T} \right)_{total} = {\frac{P_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}V_{{air}\mspace{14mu}{storage}\mspace{14mu}{unit}}}{T_{{air}\mspace{14mu}{storage}\mspace{20mu}{unit}}} = {\frac{P_{setpoint} \cdot V_{{blood}\mspace{14mu}{storage}\mspace{14mu}{{unit}/{airOT}}}}{T_{{blood}\mspace{14mu}{storage}\mspace{14mu}{unit}}} + \frac{P_{setpoinit} \cdot V_{line}}{T_{line}}}}$

Here, V_(blood storage unit/airOT) is the air volume in the bloodstorage unit at upper change-over point OT.

The control and computing unit checks during the arterial phase whetherthe above equation is satisfied. As soon as the equation is satisfied,the second time interval of the arterial phase begins, the bypass valvebeing closed. The blood pump is operated by the control and computingunit in the second time interval at the same delivery rate, until thedesired stroke volume and thus the upper change-over point OT isreached.

A leakage in the closed-off volume can lead to a change in the gasquantity enclosed in the system. If the leakage lies in theexcess-pressure region of the system, i.e. in the region of the bloodstorage unit or the adjacent line segments, the leakage leads to areduction in the enclosed air quantity, as a result of which the levelof the blood in the blood storage unit increases. There is the risk ofthe blood storage unit filling up and the blood level rising up to thehydrophobic membrane, so that the arterial phase can no longer beterminated properly. Conversely, if there is a leakage in theunderpressure region, i.e. in the gas storage unit or through air entryfrom the blood system, the enclosed air quantity is increased, so thatthe level of the blood in the blood storage unit falls. This can lead tothe blood level falling too rapidly in the venous phase, which can leadto undesired foaming, in the extreme case even to the air alarm. Thiscan however be monitored by the fact that the air quantity enclosed inthe system is observed. Such monitoring of the air quantity can beprovided for with the device according to the present invention.

The monitoring of the air quantity for the detection of a leakage takesplace by the fact that the time is detected at which the level indicator15 in blood storage unit 11 detects the level, i.e. at which the bloodreaches the preset level. The actual level of the blood is thus knownfor this time. This value is compared with the level calculated at thistime from the pressure values. If the difference between the measuredand calculated level is greater than a preset threshold value, the airquantity enclosed in the system has changed significantly, which can betraced back to a leakage in the system. In this case, the system isreinitialized. If the case of the fault occurs too frequently, centralcontrol and computing unit 27 interrupts the treatment.

An alternative evaluation makes provision such that it is not themeasured and calculated level that is taken into account, but rather thetime at which the level indicator 15 detects the preset level iscompared with the calculated time at which the preset level should beestablished. If a significant time difference is present, it isconcluded that there is a leakage in the system.

Another embodiment also makes provision to detect a leakage in thesystem in the part facing away from blood storage unit 11 behind filter23 after the increase in the level above the level detected by the levelindicator in the arterial phase, by the fact that an excessively sharplyincreasing pressure is detected by pressure sensor 16 in blood storageunit 11, whereby the pressure and/or the pressure rise per unit of timeexceeds a preset threshold value. In this case, the blood reaching thefilter leads to a pressure increase, which drastically reduces thecompliance in this part of the system. This applies in particular to thecase in which a pressure sensor is used which measures the pressure indirect contact, i.e. without a compressible intermediate space.

The invention claimed is:
 1. A device for blood treatment in asingle-needle operation, comprising: a pump configured to convey bloodin an extracorporeal blood circuit to a blood container configured tocollect blood, whereby the extracorporeal blood circuit has a bloodsupply line leading to an inlet of a blood treatment unit and a bloodreturn line departing from an outlet of the blood treatment unit, andthe blood container has a closed-off volume; a gas container having aclosed-off volume; a first connection path for connecting the bloodcontainer to the gas container in such a way that, in an arterial phase,gas expelled from the blood container during the filling of the bloodcontainer is transferred to the gas container, the first connection pathcontaining a valve configured to interrupt the connection; a secondconnection path, containing a gas compressor, for connecting the gascontainer to the blood container in such a way that, for the purpose ofemptying the blood container, gas stored in the gas container istransferred by the gas compressor to the blood container, so that bloodcan be fed to the extracorporeal blood circuit in the arterial phase andremoved from the extracorporeal blood circuit in the venous phase; and acomputing arrangement configured to determine blood volume V_(blood)enclosed in the blood container, wherein the computing arrangementincludes: a first pressure transmitter configured to measure thepressure in the blood container; a second pressure transmitterconfigured to measure the pressure in the first and second connectionpaths between the blood container and the gas compressor; and a thirdpressure transmitter for measuring the pressure in the gas container,wherein the computing arrangement is configured to calculate the bloodvolume V_(blood) on the basis of a) the measured pressure in theclosed-off volume of the blood container, b) the closed-off volume ofthe first and second connection paths between blood container and thegas compressor, and c) the closed-off volume of the as container.
 2. Theblood treatment device according to claim 1, wherein: the firstconnection path includes a bypass line bypassing the gas compressor, abypass valve being disposed in the bypass line; and the secondconnection path includes a gas line for conveying gas, the gas lineconnecting the blood container to the gas container, the gas compressorbeing disposed in the gas line.
 3. The blood treatment device accordingto claim 2, wherein the gas line includes a first line segment thatconnects the blood container to a pressure-side of the gas container,and a second line segment that connects a suction-side of the gascompressor to the gas container, a ventilation valve being provided forthe ventilation/deaeration of the first line segment.
 4. The bloodtreatment device according to claim 1, wherein the blood container has apredetermined volume and is disposed in the blood return line of theextracorporeal circuit.
 5. The blood treatment device according to claim1, wherein the gas container has a predetermined volume.
 6. The bloodtreatment device according to claim 1, wherein the pump is a blood pumpdisposed in the blood supply line.
 7. The blood treatment deviceaccording to claim 1, further comprising a control unit configured tochange over between the arterial phase and the venous phase, the controlunit configured such that the pump is operated in the arterial phase,whereby the blood container is connected via the first connection pathto the gas container, so that gas expelled from the blood containerduring the filling of the blood container is transferred to the gascontainer, and the gas compressor is operated in the venous phase,wherein the gas container is connected via the second connection path tothe blood container, so that, for the purpose of emptying the bloodcontainer, gas stored in the gas container is transferred by the gascompressor to the blood container.
 8. The blood treatment deviceaccording to claim 7, wherein the control unit is configured such that,in the arterial phase, the valve for interrupting the first connectionpath is actuated in a first time interval in such a way that the firstconnection path is not interrupted, and the valve for interrupting thefirst connection path is actuated in a second time interval in such away that the first connection path is interrupted.
 9. A device for bloodtreatment in a single-needle operation, comprising: a blood treatmentunit including an inlet and an outlet; an extracorporeal blood circuitincluding a blood supply line leading to the inlet of a blood treatmentunit and a blood return line departing from the outlet of the bloodtreatment unit; a blood container configured to collect blood in theextracorporeal circuit, the blood container having a closed-off volume;a pump configured to convey blood in the extracorporeal circuit to theblood container; a gas container configured to store gas, the gascontainer having a closed-off volume; and a computing arrangementconfigured to determine blood volume V_(blood) enclosed in the bloodcontainer, wherein the computing arrangement includes: a first pressuretransmitter configured to measure the pressure in the blood container; asecond pressure transmitter configured to measure the pressure in thefirst and second connection paths between the blood container and thegas compressor; and a third pressure transmitter for measuring thepressure in the gas container, wherein the computing arrangement isconfigured to calculate the blood volume V_(blood) on the basis of a)the measured pressure in the closed-off volume of the blood container,b) the closed-off volume of the first and second connection pathsbetween blood container and the gas compressor, and c) the closed-offvolume of the gas container, wherein the blood container is connected,via a first connection path having a valve for interrupting theconnection, to the gas container in such a way that, in an arterialphase, gas expelled from the blood container during the filling of theblood container is transferred to the gas container, and the bloodcontainer is connected, via a second connection path having a gascompressor, to the gas container in such a way that, for the purpose ofemptying the blood container in a venous phase, gas stored in the gascontainer is transferred by the gas compressor to the blood container,so that blood can be fed to the extracorporeal blood circuit in thearterial phase and blood can be removed from the extracorporeal bloodcircuit in the venous phase.
 10. The device according to claim 9,wherein: the first connection path includes a bypass line bypassing thegas compressor, a bypass valve being disposed in the bypass line; andthe second connection path includes a gas line for conveying gas, thegas line connecting the blood container to the gas container, the gascompressor being disposed in the gas line.
 11. The device according toclaim 10, wherein the gas line includes a first line segment thatconnects the blood container to a pressure-side of the gas container,and a second line segment that connects a suction-side of the gascompressor to the gas container, a ventilation valve being provided forthe ventilation/deaeration of the first line segment.
 12. The deviceaccording to claim 9, wherein the blood container has a predeterminedvolume and is disposed in the blood return line of the extracorporealcircuit.
 13. The device according to claim 9, wherein the gas containerhas a predetermined volume.
 14. The device according to claim 9, whereinthe pump is a blood pump disposed in the blood supply line.
 15. Thedevice according to claim 9, further comprising a control unitconfigured to change over between the arterial phase and the venousphase, the control unit configured such that the pump is operated in thearterial phase, whereby the blood container is connected via the firstconnection path to the gas container, so that gas expelled from theblood container during the filling of the blood container is transferredto the gas container, and the gas compressor is operated in the venousphase, whereby the gas container is connected via the second connectionpath to the blood container, so that, for the purpose of emptying theblood container, gas stored in the gas container is transferred by thegas compressor to the blood container.
 16. The device according to claim15, characterised in that the control unit is configured such that, inthe arterial phase, the valve for interrupting the first connection pathis actuated in a first time interval in such a way that the firstconnection path is not interrupted, and the valve for interrupting thefirst connection path is actuated in a second time interval in such away that the first connection path is interrupted.